CA1195346A

CA1195346A - Lightweight cement slurry and method of use

Info

Publication number: CA1195346A
Application number: CA000426068A
Authority: CA
Inventors: Lloyd B. Spangle
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1982-04-30
Filing date: 1983-04-18
Publication date: 1985-10-15
Also published as: GB8310439D0; GB2119358B; FR2526035A1; GB2119358A

Abstract

ABSTRACT OF THE DISCLOSURE

A foamed, cement slurry comprising-an aqueous hydraulic cement slurry containing gas as discrete entrained bubbles and a specific quaternary ammonium compound as a foaming agent; and a process of plugging a subterranean void by emplacing the foamed slurry in the subterranean void and then permitting the foamed slurry to therein harden.

Description

53~

LI~TWEIGHT CEMENT SLURRY AND ME~FE[OD OF USE

~he invention is a lightweight aqueous foamed cemen~ slurry and a method of using same to fill a sub-terranean void. The slurry is particularly us~ful in the art of oil and gas well cementing.
.
Cement slurries have ~een foamed as a method of lightening such slurries fo~ use in oil and gas wells or~for-subterranean grouting; U.K. Patent 819,229; ~S~E Paper No~ 75 PET-10 (1975). U.S. Patent 3,926,25't suggest~ adding a foaming agent to a cement slurEy which ~aming agent later collaborates with gas escapi~g from a gas-containing formation ~o form a foam barriex to prevent ~ubse~uent migra~ion of gas during the setting o ~he cement. It is stated ~hat the foami~g agent may be a nonio~ic, anionic, ox cationic.
The cationic surfact~nt is broadly sta~ed to be "a guat rnar~ ammonium saltl'. The only embodiment s~ecifically me~io~ed is a mi~tu~e of an o~ye~hylene aliphatic compou~d and at least one sulfate of a poly~thoxy fatty alcohol~. In U.S. Patent 4,300,633 it 28, 850Pa ~F
.

~53~6 is stated that the foaming agents to be used must be active in a highly alkaline environment and show resistance to multivalent organic cations if sea water is used. Anionic surfactants, specifically organic alkali metal sulfates or sulfonates are said to ~e particularly suitable for this purpose.

The present invention basically resides in a foamed hydraulic cement slurry comprising an aqueous slurry of hydraulic cement, entrained gas bubbles and a foaming agent selected from certain specific quaternary ammonium compound~. The cement slurry is superior to foamed cemen~ slurries prepared from other quartexnary ammonium compound~ and from o~her nonionic and anionic surfactants. When prepared with the specified foamin~
agents, a stable, uniform foam of relatively small bubble size is rapidly formed in which there is rela-tively little segregation of ~he liquid, solid and gaseous compo~ents. Hardened cement prepared from such slurries in many cases exhibit~-superior streng~h and low permeability when compared with hardened foamed ceme~ts prepared utilizing o~her foaming agents. The invention further compxises a process of cementing subterranean voids by emplacement of ~his slurry to give a strong, impermeable seal in applica~ions such as the grouting of underground bore holes and plugging the voids created or encountered during ~he drilling of oil, gas or geothexmal wells. Becaus~ of the light waight o ~he slurry, the breakdown of weak formation~
duri~g cementing o ~uch sub~erranean voids is avoided.

Foame~ slurries of ~h~ invention exhibit a~
additional ad~antage of maintaini~g their fo~med nature at temperatures in excess of about 65.6C ~150F), and 28,~50A-F -2~

even in excess of about 93.3C (200 F). At such temperatures, foamed cement slurries prepared with other than the foaming agent of this i.nvention may begin -to collapse be~ore the slurry has hardened.

The ~oamin~ agent utilized in the instant invention which permits attainment of these desirable benefits is a co~position represented by the formula R (R'-)3 N X (I) wherein X is chloride, bromide, iod.ide ox hydroxyl, R
is a aliphatic hydrocarbyl radical comprising from 8 to 16 carbon atoms and R' is, independently each occurrence, an alkyl radical of one to three carbon atoms, or hydroxyethyl, and the s~m of the carbon atoms in R and all R' is not ~reater than about 20.

More particularly, ~he invention resides in a foamed hydraulic cement sluxry comprising a gas-contalning agu~ous-slurry hydraulic cement and a foaming a~ent represented by Formula I. The in~ention further resides i~ a method of plugging a subterranean void by emplacing the foamed ceme~ slurry in the subterranean void and allowing the slurry to harden. The foamed slurry is preexab1y emplaced in a borehole drilled for an oil or gas well or in the a~nulus created between the borahole and the casing of such a well. Except for ~he eguipment adapt~d for ~he generation of the foam, as described more ~ully heraater, sta~dard oilfield equipment an~
emplacemen~ methods may be utili7!ed to carry out ~he prscess of the i~vention.

28,850A F -3-S3~
--4~

The necessary elements of the foamed cement slurry of the invention are an aqueous slurry of hydraulic cement, prepared by adding water to a hydraulic cemen-t component, which preferably comprises Portland cement, a gas which is entrained as discrete bubbles in said aqueous cement slurry and a foaming agent represe~ted by Formula I. The aqueous cement slurry, as described more fully below is commonly mixed in ~he usual fashion and thereafter the foaming agent and gas are added to the slurry in an enclosed conduit.

The most i~portant element of the foamed sl~rry of the invention is the foaming agent which is a quaternary ammonium compound represented by Eormula I, above. In Formula I, R represents a hydrocarbyl group having from 8 to 16 carbon atoms and preferably from lO
to 14 carbon atoms. R may be saturated ox un~aturated a~d;is suitably selected rom fatty alkyl groups having from 8 to 16 carbon atoms. Preferably, R is a s-traight chain hydrocarbyl radical. More ~refera~ly, R represents a mixture of hydrocarbyl radicals derived from a vegetable oil `such as cocoa oil. In the so called "coco" alkyl group~, the carbon chain length may range from 8 to 16 carbon atoms ~ith chains of lO, 12 and 14 carbon atoms predominating. Accvrdingly, R may represent a mixture of such hydrocarbyl groups ra~ging from 8 -to 16 carbon atoms or may be selected strictly fxom hydrocarbyl groups of one uniform chain l~ngth, i. 8 . the dodecyl group~

Rl represents the o~her ~hree hydrocaxbyl radicals associated with ~he guaternary nitrogen atom.
Suitably, R' is a one ~o ~hree carbon al~yl radical or a hydro~yethyl radical, independently in eac:h occurrence.

28, ~50~ 4 ~

3~

Preferably, it is an ethyl or methyl group, or is a hydroxyethyl group in up to two occurrences. More preferably, R' is hydroxyethyl in up two occurrences or is methyl. The sum of all the carbon atoms in the R
and R' groups associated with one quaternary nitrogen atom is suitably at least 12, preferably a-t least 13, and more preferably at least 14 carbon atoms and ~he sum is 20 or less, preferably l9 or les~ and more preferably 18 or less total carbon atoms.

In Formula I, ~ represents a chlorid~, bromide, iodide or hydroxyl radical; preferably it represents a chloride or hydroxyl radical and more pre~era~ly a chloride radical. Quaternary ammonium compounds of Formula I are widely known or may readily be prepared by known methods, for example by the reaction of an alkyl halide o~ the Formula RX with a ~ertiary amine of the formula (R'-)3 N under known conditions. In som instances, where the foaming agent of Formula I exhibits limited solubility in water, a cosolvent such as

2 0 iso~propanol may be added to I to permit full solubil -i~ation in the aqueous portion of the hydraulic cement slurxy~ The addition of other nonionic surfactants in conjunction wi~h the foaming agent of Formula I may serve to enhance ~he stability of ~he foam in the aqueous cement slurry. Matexials which are known foam stabilizers, such as low molecular weight polypropylene glycols of from 200 to 6CQ molecular weight, may suitably be used in addition to ~he agen~ of ~ormula I. Certai~
fa~y amine o~ides also ser~e in thi~ fa h.ion. For e~ample, bis(2-hydroxyethyl) cocoamine oxide combined wi~h ~he foaming agenk o~ I serves to fo~m a uniorm, skable foam i~ ~he agueous hyd~aulic cement slurry.
Likewise, e~Ao~ylated derivatives of the quaternary 28,850A-F -5~

9S3~
-6- ~

compound of Formula I formed by the addition of several moles of ethyle~e oxide to a compound of Forumla I
where R' is hydroxyethyl in one or two occurrences, is a suitable adjunct to be used in combination with the compound of Formula I. For example, a compound prepared by the addition of about 15 moles of ethylene oxide to cocoamine and 6ubsequent reaction with methyl chloride serves to enhance the foam st~bility of aqueous cement slurries containing the foaming agent of Formula I.

The quantity of the foaming agent I to be added to the aqueou~ hydraulic cement is sufficient to permit the major portion of the gas to r~main entrained as discrete bubbles in ~he slurry until the slurry has hardened suficiently so that coalescence or migration of bubbles is no longer possible. This ~uantl-ty will vary accordin~ to the other component~ and properties af this slurry and may be determined for a slurry by first measuring the API Standard ~PlOB "thickening time" of the slur.ry a~d then empirically determining ~h~ quan~ity of foaming age~t needed to maintain the desired stable foam~d slurry for that time under the cond.itions of temperature and pressure wh~ch will be encountered by the cemen-t slurry after it is emplaced.
To be certain ~hat the stability of the foamed slurry is maintained, it is preferable to add excess foaming agenk rather than too lit.tle. Commonly, by volume, at least ~bout 0.1 part foaming agent per 100 parts of water, preferably at least about 0.3 part and more preferably at lea~t about 0.5 part foaming agent will be ade~uate. Normally, about 3 parts, more preferably ~o mare ~ha~ about 2 part~- a~d more preferably no more ~han about 1.5 part fo~ming agent per 100 parts water will b~ adequate to serve the i~ended puxpose. ~owever 28,850A-F ~6-as noted above, excess may be added without adversely affecting the resul-ting foamed slurry.

As is readily apparent, ~he foamed cement slur~y will compxise solids, liquids (primarily water~, c~nd the entrained gas. The solid components of the cement slurry will primarily compri~e the hydraulic cement component. Preferably this comprises a Portland cement and more preferably is selected from the categories of cements of Class A, C or G, set forth in Standards of ~he American Petroleum Institute tAPI) for use in oil and gas well cementing. In addition ~o Portland cements, cements known as high alumina cements are also useful for ~his purpose.

- Powdered anhydrous sodium silicate is another solid component which may be added to the aqueous ceme~t slurry pre~ercibly in an amount of from 0.5 to 3 parts, by weight, per 100 parts o ~he hydraulic cement component. Other solid additives commonly inco.rporated i~ oil well ceme~t slurries may likewise be added so long as they do not ad~ersely affect the guality and stability of the foc~m. Such additives include fluid loss control agents, retarders, accelerators, extenders or fillers such a~ fly ash or po7~zolans ~ finely divided ~ilica as silica flour, sodium chloride, calcium chloride or sulfate, and the like. Thickeners such as bentonite a~d attapulgite, HEC, and the like are gener211y not employad in ~he inva~tion slurries. Similarly, si~ce a lightwe.ight slurry is desired, hematite, barite or other such weighting age~ts are ge~erally no~ added.

3~ Also, dispersing age~ts generally ~end to degrade or destabilize ~he foam or cause separatio~ of the liquid in solid phases, ~d for ~his reason will ordi~arily no~ be employed.

28,850A-F ~7 53~j Overall, the solids included in the foamed hydraulic cement slurry ~omprise, by volume, typically about 10 preferably at least about 15 up to about 35, preferably up to about 30 percent of the total o~med - 5 slurry.

The second major portion of the foamed cemen~
slurry is the portion made up of liquid components.
Naturally, the major portion of such liquids is water but may fur~her comprise liquid versions of the additives lQ previously mentioned above such as fluid los~ control age~ts, retarders, accelerators and sodium silicate solutions. The water may be relatively fresh water or may be an aqueous ~rine containing calcium chloride or sodium chloride, commonly produced from underground lS formation in oil and gas production or seawater.
Preerab1y, the water to be employed is relatively fresh and lacking in dissolved mineral components.
Typically, the liguid components will make up about 25, preferably at least about 30 percent up to about 60, 20 pre~erably up to about 55 percent of the total volume of ~he foamed slurry~

~ he gaseous portion of the foamed cement slurry is preferably added aæ a gas or mixture of gases to the preformed aqueous hydraulic ceme~t slurry which already contains the foaming agent. Alte.rnatively, the gas may be generated 1~ situ by ~he chemical reaction of an active metal such as aluminum or mag~esiwTI with the ~trongly basic cement slurry to ge~erate hydrogen gas. However, because o~ the explo~ive na~ure of hydrogen, such age~ts ar~- moxe ~uitably replacad by synthetic gas blowing a~ents such as are employed in ~he pla~tîc ~oam genex~ting arts. ~uitably, orga~ic nitrogen containing 28,850A~F ~8-~ 3~

compounds which generate gaseous nitrogen when decomposed are preferred as ln situ gas-generating agents. However, because of the general availabillty of compressed air, nitrogen and carbon dioxide in the oilfield, the addition of one or more of these gases to ~n aqueous hydraulic slurry containing the foaming agent is the method of choice in generating the foamed cement slurry of the invention. More prefexably, nitrogen and/or air are so employed because of ~heir lower solubility in aqueous solutions than carbon dioxide.

Gas suitably comprises, by volume, abou-t 20 percent, preferably at least abou~ 25 percent up to about 55 percent, preferably up to about 50 percent of the foamed slurry. When referred to herein, the volume 1~ o~ gas or of foamed slurry means that volume of ga~ or foam under the conditions at which the slurry will be u~ed, e. g. pressure and temperature encountered in a subterr~nean v~id at which ~he foamed slurry is emplaced.
. This generally can be de~ermined with sufficient accuracy from ~he ideal gas equation, i.e.

V2 - PlVlT2 P 2Tl It is commonly available from tables used by those who pump nitrogen or carbon dioxide in oil and gasfield opera~ions. V2 represents the volume o~ ~he gas under downhole temperature and pr ssure condi~ions, T2 and P~, and ~lVlT~ represent the pressure, volume and tem~erature of ~he gas in ~ e foamed slurry upon preparation a~ the surfac~.

2~,850A-F 9-~ ~53~

An "average diameter" of bubbles entrained in the hardened foamed slurry may be roughly approximated by cutting a statistically significant number of vertical cross~sectio~s through the hardened slurry, measuring the visible bubbles' diameters, averaging the sum of these measured diameters and then doubling that number.
A more accurate calculation of the average diameter may be obtained by multiplying the average of the measured diameters by 4/~4; i.e., by 1.27. The average diameter of the major portion of the entrained gas bubbles is preferably less ~han about 1.0 mm, more preferably less than aboutØ5 mm and most preferably less than about O.3: mm When employing the preferred foaming agents o the inve~tion, the foam produced by strongly shearing - 15 the gas-containing hydraulic cement slurry will be a highly uniform foam with bubbles of a described average diameter and with only small amounts of bubbles having a.diameter greater ~han l mm.

The foamed slurxies of the invention are suitably prepared by first mixing the base aqueous : hydraulic cement slurry with any standard ceme~t blending~
equipment, such as a paddle mixing tank or a venturi type cement slurry mixer. The means for preparing the slurxy is not a critical eleme~t of tha instant inve~tion.

Once the slurry is prepared, it is suit~bly moved by a transfer mea~s into an enclosed conduit sultable for tra~sporting fluids. Oilfield ~reati~g pipe can easily serve as suc~ a con~uit. The slurry ~ransfer means can be a co~mon hydraulic pump such as a trip}e cyli~der positive displac~ment pump commo~ly known as a "~riplexll pump. This pump i~ widely u~ed in ~he oilfield. The transfer means is ~o~ critical as 28,~50A F 10-.

long as it has the abili-ty to transport a liguid/solid slurry with suitable velocity and a centrifugal pump may likewise be employed for this purpose. To the slurry i~ the conduit is added the foaming agent which may be injected into the slurry-carrying conduit by means of a small liquid blending pump connected to the conduit by a "tee" connection or a "Y~bend" connection in a suitable fashion. It is not advisable to add the foaming agent to the slurry upstre~m from the slurry transfer means, e.g. in the mixing apparatus, since addition at that point may cause air to be entraine~ in the sl~rry making it difficult to accurately measure the ~mount of gas entrained and difficult for all but especially designed pumps to handle such a foamed mi~ture. Such pro~lems in handling may be avoided by adding the foaming agent directly at the suction o~ a pump utilized a~ a t~an~fer means or immediately down-stream from the transfer means to the conduit containing the aqueous slurry and foaming agent. The gas is then 20 suitably added at a given rate to obtain the desirPd gas.liquid:solids proportio~s for the intended application.
Ordinarily, for oilfield applications, sufficent gas is added to obtain a resulting foamed slurry of the density o~ from 600 to 1560 kg/m3 (5 to 13 pounds per gallon).
Pumping rates o~ either the slurry or of ~he gas may be adjusted so that:the desired ratio is obtained.

The foamed slurry will be genexated at the point o injectio~ of the gas and injection may be accompli~hed by a "tee" connection to generate suitable turbulence at ~he poi~t o~ mixing of ~he ga~ and ~he aqueous slurry. ~owever, it is prefer~ble to ~ur~h~r shear ~he gas~containing slurry to ob~ain a smooth, u~iform, small bubble size foam by dividing the condu1-t 28,850A-F ~11 into two separate streams and then rejoining the thus divided streams in a mixing chamber by impacting the streams against one another at a common in-line focal point. This may be accomplished by forcing the two streams throush orifices in a generally opposed fashion.
Such a dividing o the slurry stream may be accomplished by a tee in the conduit, piping from the tee through two separate conduits which are then reco~nected at ano~her tee to cause the two opposing streams to mix at the junction of the tee. The resulting fo~med slurry is then taken off as a single combined stream again.
Thi~ combined stream then is ~onducted through the well bore tubing or the annulus into the subterranean void in ~he fashion well known in th~ oilfield for cementing subterranean voids and boreholes. After being pexmitted to harden in the subterranean void, the hardened foamed cemen~ slurry often has low permeability and high strength xelative to other eguivalent density cements prepaxed from slurries other than that of the ins~ant - 20 invention. While this is the preferred me~hod of preparing the foamed slurry, o~her high shear mixing devices may be used for this purpose.

Example 1 By way of 0~ample, the following embodiments of foamed slurries of the i~vention are prepared. The cement component is an ~PI Class H cement having a surface area of about 2500 cm2/g (b~ SediGraph 5000).
The "extender" in hal of the embodiments is a fly ash (LaDue) having a surface area (also by SediGxaph) of about 3500 ~m2/g, A~h~drous sodium metasilica~e i~
pre~ent in each slurry at a level o~ about one percen~, based on weight of the c~ment component ("BWOC~).
Foaming agent of ~ormula ~I) is present at about 0.5-3 28,850~ 12-5~

percent, based on volume of water. Gas-is entrained in the foaming agent-contairling slurry prepared from the dry components and water. Resultant foamed slurry densities are described in ~he ~ollowing Table I.

T~ LE I

Percent Percent D~nsity BY Volume of Slurry Extender of Foamed Slurry Gas Water Solids BWOC in k~_3 (lb~ga]~
A 56 24.6 19.4 --- 852 (7.1) ~ 56 24.6 19.4 50 768 (6.4) B 56 32.5 11.5 --- 684 (5.7~
BB 56 32.5 11.5 50 636 (5~3) C 50 28 22 -~- 972 (8.1) CC 50 28 . ~2 50 8~4 (7.2) D 50 37 13 --- 780 (6.5) DD 50 37 13 50 720 (6.0) E 40 33.6 26.4 --- 1164 (9.7) EE 40 33.6 26.4 50 1044 (8.7) E 4Q 44.4 ~ 15.6 - 936 (7.8) FF 40 44.4 15.6 50 864 (7.23 G 30 39.2 30.8 --- 135~ (11.3~
GG 30 39.2 30.B 50 1224 (10.2) 51.8 18.2 ~-- 10~2 (9.1~
~ 30 51.8 1~.2 50 1008 (~.4) I 20 44.8 35.2 ___ 1560 (13.0) II 20 44.8 35.2 50 1392 (11.6) J 20 59.2 20.8 ~-- 12~8 (10.4) JJ 20 59.2 20.8 50 115~ ~9.6) Example 2 Utilizi~g the basic slurxy of Example lAA, various surfactants are sub~tituted as the foaming ag0nt. The base ~lurry is mixed in a Waring blender at hi~gh speed, with a special screw-on lid for th~ ble~der, ~he lid having a s~all 1.9 cm (3/4 i~ch) hole in its cent~r~ After ~h~ ~olids/w~ter blend is mi~ed or about 30 second~ to form a homoge~eous slurry, about 1~2 perce~t, b~sed on volume o~ water~ ~"BVOW") of a solution of foaming agent is injected ~hrough ~he hole in ~he 28,850A-F

.~L ~ 3 5 3L'l~

lid. The most pre~erred oaming agents of the invention generate an immediate foam which fills the blender chamber in about 2-5 seconds with a small bubble, uniform foam, having an averaye bubble diameter of 0.5 mm or less. The fo~m wh.en permitted to stand for 15-20 minutes remains unifoLm and stable without appreciable collapse or defoaming. When allowed to cure, the resultant hardened foamed cement often has permeability of about 0.1 millidarcy or less.

When less preferred foaming agents of the i~vention are substituted, from 5 20 seconds may be required to fill the chamber a~ter i~troduction of the foaming agent. The resultant foam has relatively uniform, small bubbles of an average diameter of 1 mm or less. There may be a small number of noticeably larger bubbles. The foam is fairly stable without si~nificant collapse in 15-~0 minutes. When allowed to harden, th~ resultant oamed cement often has a per-me~bility on ~he order of from 0.2 to 1 millidarcy and a cro~section of the hardened foamed cement will show only minor gravity segr~ga~ion of solids and gas bubbles.

~ hen other surfactants outside the limits o~
Fonmula I are employed as the foaming agent, the chamber may not fully fill even after l 2 mi~utes of shearing, 25 may reguire significantly mor than 3 percent ~BVOW) fc)~ni~ ager~t to fully fill the chamber wi~ foam, if at all, aIld ma~ exhibit substantial foam collapse after standing 15-20 minutes~ ~he foam ma~ contain a signi ant portion of b~bles having average diame~er 30 qreater than o~e mm and the hardened resultant cement, if remain~ng foamed at all, may exhibit sevexe segreg~-tlor and stratiie::ation of bubbles and solids resulting in 28, 850P~F ~14;

~553~

highly permeable portions in the upper part and dense, impermeable portions in the lower part.

Example 3 A cement slurry is prepared from a 35:65 (weight) pozzolan:Class G cement blend with sufficient water to make about a 1680 kg/m3 (14 pound per gallon) slurry. About 59.2 m3 (370 barrels) of the slurry is prepared with the addition of about 1 percent of anhydrous sodium metasilicate, about 18 percent sodium chlsride and about 0.7 percent of a calcium lignosulfonat~
re~arder (all BWOC). To ~he slurry so prepared is added abou~ 1.5 percent (BVOW) of a foaming agPnt comprising ~by vol-ume) about 3 parts trimethylcocoammonium chloride and about 1 part bis(2-hydro~yethyl) cocoamine o~ide, in about an equal volume o iso-propanol. To this 59.2 m3 (370 barxels) of slurry co~taining -the ~oamin~ agent is added about 3500 m3 (125,000 standard cubic f~e~) of nitrogen by iniecting through a "tee" to the line containing the slurry. The lin~ is thereafter ~0 split into 2 lines at the "tee" and ~he 2 line~ are later reco~binad at another "tee" through a choke device in each of khe lines causing the 2 slurry streams to impact with co~siderable kurbulence and good mi~ing, as described ~arlier. The rate of nitrogen addition to the slurry i5 varied, in a staged fashion, throughout wi~h more being added to slurry in~ended for the bottQm o the hole a~d less to slurry added to ~he top. In ~his a~hion, a slurry of relatively uniform densi~y o about 1260 kgJm3 (10.5 ppg~ rom top to bottom of the hole i5 obtained. The cem~nt slurries are used to ceme~t a fi~e and o~e-half inch ca~ing to a depth of abou~ 2490 m (~300 feet) in a previously ~rilled borehole.
~bout 8 m3 (50 barrels3 of a gelled æpacer ~lui~ is 28,250A~F -15 ~ 6-first pumped into -the casing followed by about 4.8 m3 (30 barrels) of an aqueous surfactant wash solution.
Then about 3.84 m3 (24 barrels) of the 1680 kg/m3 (14 ppg) base slurry follows ater which the remaining 59.2 m3 (370 barrels) of slurry to which the nitrogen has been added are pumped, followed by about 14.4 m3 (90 barrels) of a 36:65 pozzolan:Class G tail-in slurry made up to about 1800 kg/m3 (15 ppg). About 30.4 m3 (190 barrels) of salt water is then pumped until returns of the spacer fluid and chemical wash are seen. The well bore is then shut in and the cement slurries are permitted to set. In ~his manner, a "long-string"
casing job is completed in one single pumping operation without ha~ing to "stage" the different cement slurries over the desired intervals o the borehole.
..
Instead of varyin~ the gas addition, the yas may be metered into ~he slurry at abou~ a constan-t rate. The resulting slurry will be of greater density near -the bottom of ~he wellbore when in place and of lesser density near ~he tQp. The density of the foamed slurr~ being pumped into the wellbore can be monitored by an instrument commonly used in oilfield service called a densiometer.

28,850A~F 16

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A foamed hydraulic cement slurry com-prising an aqueous slurry of hydraulic cement, an entrained gas in the form of multiple, discrete bubbles and a foaming agent represented by the formula R(R'-)3 N+ X - wherein X is chloride, bromide, iodide or hydroxyl, R is an aliphatic hydrocarbyl radical comprising from 8 to 16 carbon atoms and R1 is, independently each occurrence, an alkyl radical of one to three carbon atoms or hydroxyethyl, and the sum of the carbon atoms in R and all R' is not greater than about 20.

2. The foamed slurry of Claim 1 wherein, by volume, the solids therein comprise from 10 to 35 percent of the foamed slurry, under conditions of use.

3. The foamed slurry of Claim 1 wherein, by volume, the gas comprises from 20 to 55 percent of the foamed slurry, under conditions of use.

4. The foamed slurry of Claim 1, 2 or 3 wherein the hydraulic cement component comprises a Portland cement.

5. The foamed slurry of Claim 1 wherein the solids and gas, by volume, comprise from 10 and 20, respectively to 35 and 55, respectively, percent of the foamed slurry, under conditions of use.

6. The foamed slurry of Claim 1 wherein the foaming agent is present in an amount sufficient to permit the major portion of said gas to remain as discrete bubbles until said slurry has hardened.

7. The foamed slurry of Claim 1 wherein the foaming agent is present, by volume, in an amount of from 0.1 part to 3 parts per 100 parts water.

8. The foamed slurry of Claim 7 wherein the foaming agent is present in an amount of from 0.5 to 1.5 parts per 100 parts water and the average diameter of said bubbles is about 0.5 millimeter or less.

9. The foamed slurry of Claim 1 wherein the aqueous slurry further comprises, by weight, from 0.5 part to 3 parts sodium silicate per 100 parts of the hydraulic cement component.

10. The foamed slurry of Claim 9 wherein the hydraulic cement component comprises a Portland cement.

11. The foamed slurry of Claim 9 which further comprises particles of finely divided fly ash having surface area of about 3000 cm2/g or greater and the surface area of the hydraulic cement component is about 2000 cm2/g or greater.

12. The foamed slurry of Claim 6 wherein the foaming agent R is a hydrocarbyl radical of an average chain length of from 10 to 14 carbon atoms, R' is, independently each occurrence, methyl, ethyl, or in up to two occurrences hydroxy-ethyl, and X is chloride.

13. The foamed slurry of Claim 9 wherein the foaming agent R is a hydrocarbyl radical of an average chain length of from 10 to 14 carbon atoms, R' is, independently each occurrence, methyl, ethyl, or in up to two occurrences hydroxyethyl, and X is chloride.

14. The foamed slurry of Claim 12 wherein the hydraulic cement component comprises a Portland cement.

15. The foamed slurry of Claim 6 wherein the foaming agent is selected from trimethylcocoammonium chloride, methyl bis(2-hydroxyethyl) cocoammonium chloride, trimethyldodecylammonium chloride or com-binations thereof.

16. The foamed slurry of Claim 15 wherein the foaming agent is present, by volume, in an amount of from 0.5 to 1.5 part per 100 parts water and the hydraulic cement component comprises a Portland cement.

17. The foamed slurry of Claim 15 wherein the foaming agent is present, by volume, in an amount of from 0.5 to 1.5 part per 100 parts water, the hydraulic cement component comprises a Portland cement, the average diameter of said bubbles is about 0.5 millimeter or less and the foaming agent is selected from trimethylcocoammonium chloride and methyl bis(2-hydroxyethyl) cocoammonium chloride.

18. The foamed slurry of Claim 17 which further comprises from 0.5 part to 3 parts sodium silicate per 100 parts, by weight, of the Portland cement component.

19. The foamed slurry of Claim 17 which further comprises bis(2-hydroxyethyl) cocoamine oxide.

20. The foamed slurry of Claim 18 which further comprises bis(2-hydroxyethyl) cocoamine oxide in an amount of 1 part, by volume, per 3 parts of the foaming agent, and the foaming agent is trimethyl-cocoammonium chloride.

21. The foamed slurry of Claim 1 which further comprises finely divided silica.

22. The foamed slurry of Claim 17 which further comprises finely divided silica.

23. A process for plugging a subterranean void with a lightweight, relatively impermeable plugging agent comprising emplacing a foamed slurry of Claim 1 or 5 in the void and permitting same to harden.

24. A process for plugging a subterranean void with a lightweight, relatively impermeable plugging agent comprising a foamed slurry of Claim 6 in the void and permitting same to harden.

25. A process for plugging a subterranean void with a lightweight, relatively impermeable plugging agent comprising emplacing a foamed slurry of Claim 8 in the void and permitting same to harden.

26. A process for plugging a subterranean void with a lightweight, relatively impermeable plugging agent comprising emplacing a foamed slurry of Claim 9 in the void and permitting same to harden.

27. A process for plugging a subterranean void with a lightweight, relatively impermeable plugging agent comprising emplacing a foamed slurry of Claim 20 in the void and permitting same to harden.

28. A process for plugging a subterranean void with a lightweight, relatively impermeable plugging agent comprising emplacing a foamed slurry of Claim 21 in the void and permitting same to harden.

29. The process of Claim 24 wherein the foamed slurry is subjected to a temperature of about 150° F or greater prior to hardening.

30. The foamed slurry of Claim 6 wherein the foaming agent is present in an amount sufficient to permit the major portion of said gas to remain as discrete bubbles at a temperature of about 150° F or greater until said slurry has hardened.